This invention relates generally to radar operations, and more specifically to, systems and methods for addressing radar fading problems which can occur during hovering operations.
Flight platforms must be able to hold an altitude during hovering operations. An example of such a flight platform is a helicopter and an example of such an operation is a rescue maneuver. Radar altimeters are commonly implemented within such flight platforms as part of an overall flight control system. Some of these flight platforms utilize the flight control systems to maintain hovering altitudes, and these flight control systems rely on reliable radar altimeter performance during the hover operations.
A radar altimeter typically includes a transmitter for applying pulses of electromagnetic energy at regular intervals to an antenna which then radiates the energy, in the form of a transmit beam, towards the earth""s surface. A transmit beam from a radar is sometimes said to xe2x80x9cilluminatexe2x80x9d an area (e.g. the ground) which reflects (returns) the transmit beam. The reflected beam (ground returns) is received at a receive antenna of the radar altimeter and processed to determine an altitude.
Ground return fading due to phase cancellations in radar return signals is a common occurrence during hovering operations. A ground return fade occurs when a patch of ground reflecting radar transmit pulses actually reflects a multitude of radar returns whose net phase adds to zero, resulting in a total radar return amplitude of zero. When an aircraft is moving horizontally at low altitudes, these ground return fades and opposite amplitude peaks occur rather randomly and quickly such that an integrated or filtered net result typically has no affect on radar altimeter performance.
However, when in a hovering or other holding altitude operation, with little or no horizontal movement, a ground return fade can last indefinitely. At least one result is that the radar altimeter can break track, providing an indeterminate altitude to the flight control system.
In one aspect, a method for suppressing ground return radar fading in a radar altimeter is provided. The method comprises providing a radar gate width which corresponds to an area that is smaller than an antenna illumination area being illuminated by transmissions from the radar altimeter, dithering the radar gate viewing area within the antenna illumination area being illuminated by the transmissions from the radar altimeter, and taking radar return samples with the radar altimeter.
In another aspect, a radar altimeter configured to suppress ground return fading is provided. The radar altimeter comprises a sequencer for modulating a first signal, a transmitter coupled to the sequencer for transmitting a radar signal including the modulated first signal toward the ground, and a receiver for receiving a reflected radar signal from the ground. The received radar signal includes the modulated first signal. The radar altimeter further comprises a sampling clock, a dithering circuit coupled to the sampling clock and a digitizer coupled to the receiver and the sampling clock. The digitizer generates digital samples of the modulated first signal. The sampling clock, the dithering circuit, and the digitizer comprise at least a portion of a radar range gate function providing a radar range gate width which corresponds to a viewing area that is smaller than an antenna illumination area where the transmitter transmits the radar signal and dithering a position of the radar range gate within the illumination area where the transmitter transmits the radar signal.
In still another aspect, a unit for dithering a position of a radar range gate is provided. The unit comprises a sampling clock, a dithering circuit coupled to the sampling clock and an analog-to-digital (A/D) converter coupled to the dithering circuit. The AID converter takes samples following each transmission of a radar altimeter. The times when the samples are taken being dithered from the sampling clock cycles by the dithering circuit.
In yet another aspect, a method for operating a radar altimeter is provided. The method comprises transmitting a signal towards the ground, receiving a reflection of the transmitted signal within a radar range gate, the radar range gate having a width corresponding to a viewing area on the ground smaller than an area reflecting the transmitted signal, and moving a position of the radar range gate within the area reflecting the transmitted signal. The method further includes repeating the above described steps.